1. Field of the Invention
The present invention relates to optical projection and more particularly concerns an improved liquid crystal light valve color projection system.
2. Description of Related Art
Liquid crystal light valve (LCLV) projection systems find utility in many applications, such as those where a very large projection screen must be illuminated by a projector occupying a very small volume, and also in very high brightness projection display systems. Where intensity of light emitted by a conventional display screen, such as a cathode ray tube, is not adequate because of high ambient light conditions, or where very large projection screens are employed in limited space, use of a liquid crystal color display projection system may be advantageous. Some prior color projection systems embody production of three high intensity substantially monochromatic light beams of different color that are fed to or through three separate liquid crystal modulators. Where transmission type (active-matrix) liquid crystal light valves are employed, each liquid crystal light valve individually modulates its respective monochromatic beam over a spatial array of pixels, and the beams are then combined within a multiplexer or combining prism and projected as a single combined beam of appropriate color. The combined beam bears information imposed thereon by computer controlled modulation of the several liquid crystal light valves. The combined beam is fed through a projection lens onto the back surface, in some cases, of a diffuse display screen to provide appropriate display on the screen front surface.
Active-matrix liquid crystal projectors have been made by Victor Corporation of Japan (JVC), Seiko Epson Company, Lt., Sharp, Sanyo and Toshiba, among others, and have been provided as a substitute for conventional cathode ray tube display systems. To achieve a high definition full resolution color picture on screen, the Victor Corporation and Seiko Epson Company liquid crystal projector, identified as Model L-1000, employs a 250 watt halogen lamp, three transmissive liquid crystal display panels and a complex system of lenses and dichroic mirrors. The dichroic mirrors separate white light emitted by the high intensity halogen lamp into three monochromatic beams, red, blue and green. These are passed through the individual liquid crystal modulator panels, and the resulting modulated monochromatic light beams are combined into a single multi-color beam by an X-prism, and then projected through a set of lenses onto the screen. Such systems necessarily employ large numbers of optical relay systems or optical elements, such as lenses and mirrors, to split and distribute reading light from the source lamp to the various liquid crystal modules. Further, these relay systems require significant amounts of space to position the several dichroic reflecting mirrors, which in and of themselves require optical relay systems to provide longer focal lengths that enable proper positioning of such additional components. Physical size of the system is therefore greatly increased.
Similar problems, e.g. large numbers of components and increased space requirements, exist in those projection systems employing reflective liquid crystal light valves. In some reflective liquid crystal light valve systems reading light from a high powered source is polarized by a polarizing beam splitting prism designed to reflect "S" polarized light (where the polarization axis, namely the E field vector, is parallel to the plane of incidence) and to transmit "P" polarized light (where the polarization axis, the E field vector, is perpendicular to the plane of incidence). The "S" polarization component of the reading light is reflected to the light valve, which, when activated by an image from a writing light source such as a cathode ray tube, reflects the polarized light and rotates its polarization 90.degree. so that it becomes "P" polarized light. The "P" polarization component is transmitted through the prism to the projection lens. When such reflective liquid crystal light valves are employed in a color projection system, the input light path must be lengthened by additional relay optics to provide for separation of the reading light source into three color components and pre-polarization of the several color components.
Other prior art systems include projector configurations having three projection lenses, projectors having multiple color tuned polarization analyzers or color trim filters, and dichroics requiring oil emersion. Some systems require light of both linear polarizations. The prior art tends to be more costly and bulky, owing to the larger number of components. Oil emersion of dichroics, if required, degrades efficient reflection of P state polarization. Use of systems employing three projection lenses requires precision convergence at the screen, and therefore requires readjustment for reconvergence whenever the projector or screen is moved. Further, in such three projector systems, trapezoid correction is different for each color when the projector is tilted with respect to the screen.
Accordingly, it is an object of the present invention to provide a color projection system which avoids or minimizes the above mentioned problems.